The present invention relates to modified aromatic polyisocyanates and to the production of rigid roamed materials using such polyisocyanates which show a good compatibility towards alkanes used as blowing agents. The use of specially modified distillation bottoms from tolylene diisocyanate production is particularly preferred.
1. Field of the Invention
As is generally known, distillation bottoms are obtained in the production of tolylene diisocyanate ("TOI") which, in addition to monomeric TDI, contain various reaction products or secondary products, e.g. carbodiimides, uretoneimines, isocyanurates, uretdiones, and carbamides. This TDI residue adversely affects the economics of the production process and causes considerable technical problems relating to the thermal recycling of the distillation bottoms. Moreover, the disposal of distillation residues, such as these, on land-fill sites constitutes a severe threat to the environment.
Processes have recently become known which effect an improvement in the TOI yield and thus a reduction in the TOI bottoms (German Offenlegunschrift 2,850,60). A further process is described in East German Patent 257,827, in which a large part of the monomeric TDI is recovered from the bottoms by the addition of isocyanates of the diphenyl methane series ("MDI") to the TOI residue and subsequent distillation under vacuum. The residual content of free TDI is thus in the region of about 1%. In addition, an improvement in the yield of TOI monomer is obtained in this process by means of acid acceptors, wherein chlorine compounds of TDI are cracked. The residual fraction comprising the TDI bottoms and the MDI residues is recommended for the production of polyurethanes. However, there is no information as to how the residual fraction should be processed. Tests have shown that a polyisocyanate mixture such as this, comprising TDI and MDI fractions (a polyaromatic polyisocyanate mixture), can in principle be foamed using polyols and known halogen-containing foaming agents. However, rigid polyurethane foams are produced which are very brittle and which crumble easily, and which cannot be used in practice.
This is explainable, since the double thermal processing of the isocyanates (i.e., the mixture of non-distillable TDI polymer residues and MDI with its highly variable composition) can give rise to secondary products in the TDI and to compounds which are formed from the two isocyanates TDI and MDI, so that rigid polyurethane foams which meet the standard requirements cannot be obtained. The severe thermal processing conditions lead to a dark brown/grey foam coloration, which does not correspond to the state of the art.
To improve the properties of rigid polyurethane foamed materials required for industrial application thereof, monomeric or polymeric isocyanates of TDI and also of the MDI series have previously been modified with polyols based on ethers or esters.
When reaction of the above-mentioned polyisocyanate mixtures with ethylene oxide ("EO"), propylene oxide ("PO") and/or butylene oxide-polyether polyols based on TMP or propylene glycol to form urethane-containing polyisocyanates was attempted, a slight improvement in the foamed materials produced therefrom did result. The foam materials still did not correspond to the industry's requirements.